The idea of a
mail exchanger is probably new to many of you, so let's go over it in
a little more detail. A simple analogy should help here: imagine that
a mail exchanger is an airport, and instead of setting up
MX
records to instruct mailers where to send
messages, you're advising your in-laws on which airport to fly into
when they come visit you.

Say you live in Los Gatos, California. The closest airport for
your in-laws to fly into is San Jose, the second closest is San
Francisco, and the third Oakland. (We'll ignore other factors like
price of the ticket, Bay Area traffic, etc.) Don't see the parallel?
Then picture it like this:

The
MX
list is just an ordered list of
destinations that tells mailers (your in-laws) where to send messages
(fly) if they want to reach a given domain (your house). The
preference value tells them how desirable it is to use that
destination - you can think of it as a logical
"distance"
from the eventual destination (in any units
you choose), or simply as a
"top-ten"
-style ranking of
the proximity of those mail exchangers to the final
destination.

With this list, you're saying,
"Try to fly into San Jose,
and if you can't get there, try San Francisco and Oakland, in that
order."
It
also
says that if you reach San
Francisco, you should take a commuter flight to San Jose. If you wind
up in Oakland, you should try to get a commuter to San Jose, or at
least to San Francisco.

What makes a good mail exchanger, then? The same qualities that make a
good airport:

Size

You wouldn't want to fly into tiny Reid-Hillview Airport to get
to Los Gatos, because the airport's not equipped to handle large
planes or many people. (You'd probably be better off landing a big jet
on Highway 280 than at Reid-Hillview.) Likewise, you don't want to use
an emaciated, underpowered host as a mail exchanger; it won't be able
to handle the load.

Uptime

You know better than to fly through Denver International Airport
in the winter, right? Then you should know better than to use a host
that's rarely up or available as a mail exchanger.

Connectivity

If your relatives are flying in from far away, you've got to make sure
they can get a direct flight to at least one of the airports in the
list you give them. You can't tell them their only choices are San
Jose and Oakland if they're flying in from Helsinki. Similarly, you've
got to make sure that at least one of your hosts' mail exchangers is
reachable to anyone who might conceivably send you mail.

Management and administration

How well an airport is managed has a bearing on your safety while
flying into or just through the airport, and on how easy it is to
use. Think of these factors when choosing a mail exchanger. The
privacy of your mail, the speed of its delivery during normal
operations, and how well your mail is treated when your hosts go down
all hinge upon the quality of the administrators who manage your mail
exchangers.